Internet Engineering Task Force Matt Crawford
INTERNET-DRAFT Fermilab
<draft-ietf-ipngwg-fddi-ntwrks-01.txt> November 7, 1995
A Method for the Transmission of IPv6 Packets over FDDI Networks
Status of this Memo
This document is an Internet Draft. Internet Drafts are working
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Distribution of this memo is unlimited.
Introduction
This memo specifies the MTU frame format for transmission of IPv6
[IPV6] packets on FDDI networks, including a method for MTU
determination in the presence of 802.1d bridges to other media. It
also specifies the method of forming IPv6 link-local addresses on
FDDI networks and the content of the Source/Target Link-layer Address
option used the the Router Solicitation, Router Advertisement,
Neighbor Solicitation, and Neighbor Advertisement messages described
in [DISC], when those messages are transmitted on an FDDI network.
Maximum Transmission Unit
FDDI permits a frame length of 4500 octets (9000 symbols), including
at least 22 octets (44 symbols) of Data Link encapsulation when
long-format addresses are used. Subtracting 8 octets of LLC/SNAP
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header, this would, in principle, allow the IPv6 packet in the
Information field to be up to 4470 octets. However, it is desirable
to allow for the variable sizes and possible future extensions to the
MAC header and frame status fields. The default MTU size for IPv6
packets on an FDDI network is therefore 4352 octets. This size may
be reduced by a Router Advertisement [DISC] containing an MTU option
which specifies a smaller MTU, or by manual configuration of a
smaller value on each node. If a Router Advertisement is received
with an MTU option specifying an MTU larger than the default or the
manually configured value, that MTU option may be logged to system or
network management but must be otherwise ignored.
Frame Format
FDDI provides both synchronous and asynchronous transmission, with
the latter class further subdivided by the use of restricted and
unrestricted tokens. Only asynchronous transmission with
unrestricted tokens is required for FDDI interoperability.
Accordingly, IPv6 packets shall be sent in asynchronous frames using
unrestricted tokens. The robustness principle dictates that nodes
should be able to receive synchronous frames and asynchronous frames
sent using restricted tokens.
IPv6 packets are transmitted in LLC/SNAP frames, using long-format
(48 bit) addresses. The data field contains the IPv6 header and
payload and is followed by the FDDI Frame Check Sequence, Ending
Delimiter, and Frame Status symbols.
+-------+ ^
| FC | |
+-------+-------+-------+-------+-------+-------+ |
| Destination FDDI address | |
+-------+-------+-------+-------+-------+-------+ FDDI
| Source FDDI address | header
+-------+-------+-------+-------+-------+-------+ |
| DSAP | SSAP | CTL | OUI | |
+-------+-------+-------+-------+-------+-------+ |
| Ethertype | v
+-------+-------+-------+-------+-------+------+------+
| IPv6 header and payload ... /
+-------+-------+-------+-------+-------+------+------+
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FDDI Header Fields:
FC The Frame Code shall be in the range 50 to 57 hexadecimal,
inclusive, with the three low order bits indicating the
frame priority. Transmission by hosts and routers of frames
with priority zero (FC =3D 50 hex) is discouraged in order to=
aid implementors who wish to distinguish on-ring nodes from
nodes which lie behind bridges.
DSAP, SSAP Both the DSAP and SSAP fields shall contain the value AA
hexadecimal, indictating SNAP encapsulation.
CTL The Control field shall be set to 03 hexadecimal, indicating
Unnumbered Information.
OUI The Organizationally Unique Identifier shall be set to
000000 hexadecimal.
Ethertype The ethernet protocol type ("ethertype") shall be set to the
value 86DD hexadecimal.
Interaction with Bridges
For correct operation when mixed media are bridged together, nodes
transmitting IPv6 on FDDI must implement the following simple
mechanism for bridge detection. If a node receives a valid Router
Advertisement or Neighbor Advertisement on an FDDI interface in a
frame with priority zero (FC =3D 50 hexadecimal), the link MTU used by=
that node on that interface shall not exceed 1500 octets. The event
of detecting a mixed-media bridged network should be logged to system
management the first time it occurs after interface initialization,
and such logging should include the source IPv6 address of the packet
which triggered bridge detection.
Nodes transmitting IPv6 on FDDI must provide a configuration option
to disable the bridge detection mechanism. This option may be used
when the presence of a non-conforming implementation is known. By
default, bridge detection must be enabled.
It is possible that a medium other than Ethernet may be bridged to
FDDI, and that the correct MTU will be neither 1500 nor 4352 octets.
In such a case, bridge detection should be disabled and routers
should be configured to advertise the correct MTU.
The only contemplated use of the LLC priority field of the FC octet
is to aid in link MTU determination. It would be sufficient for that
purpose to require only that Router Advertisements and Neighbor
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Advertisements sent on FDDI always have non-zero priority. However,
it may be simpler or more useful to transmit all IPv6 packets on FDDI
with non-zero priority.
Stateless Autoconfiguration and Link-Local Addresses
The address token [CONF] for an FDDI interface is the interface's
built-in 48-bit IEEE 802 address, in canonical bit order and with the
octet in the same order in which they would appear in the header of
an ethernet frame. (The individual/group bit is in the first octet
and the OUI is in the first three octets.) A different MAC address
set manually or by software should not be used as the address token.
An IPv6 address prefix used for stateless autoconfiguration of an
FDDI interface must be 80 bits in length.
The IPv6 Link-local address [AARCH] for an FDDI interface is formed
by appending the interface's IEEE 802 address to the 80-bit prefix
FE80::.
+-------+-------+-------+-------+-------+-------+------+------+
| FE 80 00 00 00 00 00 00 |
+-------+-------+-------+-------+-------+-------+------+------+
| 00 00 | FDDI Address |
+-------+-------+-------+-------+-------+-------+------+------+
Address Mapping -- Unicast
The procedure for mapping IPv6 addresses into FDDI link-layer
addresses is described in [DISC]. The Source/Target Link-layer
Address option has the following form when the link layer is FDDI.
+-------+-------+-------+-------+-------+-------+------+------+
| Type |Length | FDDI Address |
+-------+-------+-------+-------+-------+-------+------+------+
Option fields:
Type 1 for Source Link-layer address.
2 for Target Link-layer address.
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Length 1 (in units of 8 octets).
FDDI Address
The 48 bit FDDI IEEE 802 address, in canonical bit order.
Address Mapping -- Multicast
An IPv6 packet with a multicast destination address DST is
transmitted to the FDDI multicast address whose first two octets are
the value 3333 hexadecimal and whose last four octets are the last
four octets of DST, ordered from more to least significant.
+-------+-------+-------+-------+-------+-------+
| 33 | 33 | DST13 | DST14 | DST15 | DST16 |
+-------+-------+-------+-------+-------+-------+
Security Considerations
Security considerations are not addressed in this memo.
References
[AARCH] R. Hinden, S. Deering, IP Version 6 Addressing Architecture.
Currently draft-ietf-ipngwg-addr-arch-03.txt.
[CONF] S. Thomson, IPv6 Stateless Address Autoconfiguration. Currently
draft-ietf-addrconf-ipv6-auto-05.txt.
[DISC] T. Narten, E. Nordmark, W. A. Simpson, Neighbor Discovery for IP
Version 6 (IPv6). Currently draft-ietf-ipngwg-discovery-02.txt.
[IPV6] S. Deering, R. Hinden, Internet Protocol, Version 6 (IPv6)
Specification. Currently draft-ietf-ipngwg-ipv6-spec-02.txt.
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Author's Address
Matt Crawford
Fermilab MS 368
PO Box 500
Batavia, IL 60510
USA
Phone: +1 708 840-3461
EMail: crawdad@fnal.gov
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